1. Field of the Invention
The present invention relates to an electrophotographic apparatus, such as a printer, a copier or a facsimile machine, and relates in particular to an electrophotographic apparatus that includes exposure means for exposing a photosensitive body to digital light.
2. Description of the Related Background Art
Electrophotographic printers have been drawing attention because of the features, such as high image quality and high printing speeds, that they provide.
FIG. 21 is a diagram illustrating the schematic arrangement of an electrophotographic printer that employs light beams for scanning and for exposing.
An organic or inorganic electrophotographic photosensitive body 1, a rotating drum, is a recording medium, and is rotated clockwise, as indicated by an arrow, at a predetermined peripheral speed.
Charging means 2 is a corona charging device or a contact charging roller, for example, that uses a predetermined polarity and potential to uniformly charge the external surface of the rotating photosensitive body 1.
An optical scanning apparatus 3 is a laser scanner or an LED array apparatus (a light-emitting device array apparatus), for example, that serves as image forming light irradiation means. When a light beam L is modulated and is output, in consonance with image information, by the optical scanning apparatus 3, it is used to scan and expose the charged face of the rotating photosensitive body 1. Thereafter, on the portion of the bright face of the photosensitive body 1 that is exposed the potential is decayed, and due to the potential contrast subsequently established by the potential on the dark exposed portion, an electrostatic latent image is formed on the face of the photosensitive body in consonance with the image information pattern that is scanned and exposed.
Development means 4 uses a developer (toner) to develop normally or to develop reversely the latent electrostatic image on the photosensitive body 1 and to visualize it as a toner image.
Transfer means 5 is a transfer corona charging device or a transfer roller, for example. A transfer material (a recording medium) P is fed by a paper feeding mechanism (not shown) to a transfer portion between the transfer means and the photosensitive body 1 in consonance with a predetermined control timing. The transfer material P is conveyed while attached to the face of the photosensitive body 1, while concurrently the toner image on the photosensitive body 1 is transferred to the face of the transfer material P.
Once the transfer material P has passed through the transfer portion it is separated from the photosensitive body 1 and is fed into fixing means 7, wherein, the toner image is subjected to a fixing process and the transfer material P is printed out.
After the transfer material P has been separated from the photosensitive body 1, residual toner is removed from the face of the body 1 by cleaning means 6, such as a blade or a fur brush, and the cleaned face of the photosensitive body 1 is again employed for image formation.
The main light sources for the image forming light irradiation means 3 are a laser and an LED. The procedure performed to scan the photosensitive body 1, which is a recording medium, greatly affects printer performance, including the quality of the images it provides and the printing speed. Mainly, an LED is employed for processing involving the use of a spatial installation and electric scanning combination, while a laser is employed for processing involving the use of an optical scanning and electric scanning combination.
Further, while taking the relationship between image information and an exposure portion into account, roughly two image forming methods are available for use with a digital electrophotographic apparatus.
One method is an image exposure method (hereinafter referred to as the IAE method) according to which an image portion is exposed, and the other method is a background exposure method (hereinafter referred to as the BAE method) according to which a non-image portion (the background) is exposed.
The BAE method is the same as the image forming method used for an analog electrophotographic apparatus, and as an advantage provides that development and cleaning properties and a developer can be used in common by an analog electrophotographic apparatus.
On the other hand, according to the IAE method, to obtain an image, reversal-developing using a developer having the opposite polarity must normally be performed.
In practice, both methods are used, but their use tends to be determined by the operating limitations imposed by the photosensitive body and the developer that are employed.
Various development methods, including one-component development and two-component brush development, have been proposed or adapted for use with the development means 4 in accordance with the needs for specific processing, such as monochromatic printing or color printing. Generally, the image reproduction property of two-component brush development is superior to that of one-component development. These methods possess unique properties.
The following is a listing of the properties possessed by the primary developing methods.
a) BMT method and FEED method PA0 b) touch-down method PA0 c) jumping method PA0 d) projection method PA0 e) magnedynamic method PA0 f) IMB method PA0 g) CMB method
one-component, insulating, magnetic, contact PA1 the FEED method especially has substantially the same image characteristics as the two-component brush development. PA1 one-component, insulating, nonmagnetic, contact PA1 the occurrence of fog due to contact development is a problem. PA1 one-component, insulating, magnetic, noncontact PA1 because of noncontact, fog and scratches seldom occur. PA1 one-component, insulating, nonmagnetic, noncontact PA1 because of noncontact, fog and scratches seldom occur, and because this method is nonmagnetic, color printing is available. PA1 one-component, conductive, magnetic, contact PA1 induced charging using a latent image electric field, brush development PA1 either a positive or a negative latent image can be developed, but image transfer is difficult. PA1 two-component, insulating, nonmagnetic, contact PA1 since an insulating carrier is employed, electric charges of the opposite polarity are accumulated after image development. PA1 reproduction of solid image portions is not especially notable, but reproduction of fine lines is superior. PA1 two-component, conductive, nonmagnetic, contact PA1 since a conductive carrier is employed, electric charges of the opposite polarity are not accumulated after image development. PA1 reproduction of solid image portions is superior, but reproduction of fine lines at a low density is not especially notable.
The transfer and separation performances are greatly affected by the transfer efficiency and the separation/re-transfer latitude. Since in the IAE method the potential of the non-image portion (the background) is higher than the potential of the image portion, the latitude for the BAE method is greater than that for the IAE method.
Further, since the potential of the photosensitive body has decayed by the time cleaning is initiated, according to the IAE method for the development of an image on a low potential portion, much developer tends to remain attached to the photosensitive body at the cleaning position, and as for the cleaning property, the latitude for the BAE method is also greater than that for the IAE method.
As is apparent from the above explanation, the BAE method is easier to design, and has the potential of providing a stable electrophotographic apparatus for which the latitude is large.
However, for image recording for which a light beam is used for scanning, the latitude for the BAE method is less than the latitude for the IAE method, as will be described below.
Specifically, according to the image recording technique for which light beam scanning is used, the size, the shape and strength of a light beam spot greatly affect image quality and stabilization. Especially with an electrophotographic apparatus, since the uniform surface potential distribution that is obtained on the face of a photosensitive body by charging is selectively decayed by irradiation with a light beam, the size and the shape of the light beam spot affects the distribution.
According to the IAE method, the surface potential of a photosensitive body is decayed by exposing the image area (solid black portion) to a light beam, and the developer is attached to the portion whereat the potential has been reduced. Therefore, when a half value width Wv (F.W.H.M.; Full Width at Half Maximum) of a latent image potential distribution is increased relative to the pixel width, the line width of a character or a line is increased to print the character or the line as a solid color.
According to the BAE method, surface potential is decayed by exposing the background (nonblack portion) of a photosensitive body to a light beam, and the area whereat the developer is to be attached is an area for which the potential has not decayed, i.e., where the surface potential is high. Therefore, when the half value width of the latent image potential distribution is increased relative to the pixel width, the line width of a character or a line is reduced and the color of the character or the line is faded.
Therefore, for these image forming methods, there is an upper limit for the spot diameter and strength of a light beam.
In FIG. 22, the state of one line in accordance with the IAE method, i.e., the light beam ON state for only one line, is shown on the left, and the state of one line in accordance with the BAE method, i.e., the light beam OFF state for only one line, is shown on the right. The latitude for the IAE method is VD-Vi, and the latitude for the BAE method is Vb-V2.
As is apparent from FIG. 22, in accordance with the BAE method, if the spot diameter is too small or the strength of the light beam is insufficient relative to the scanning interval, a potential gap is generated in the light beam irradiated portion, and V2 is increased while the latitude is reduced. Thus, there also is a lower limit for the spot diameter and the strength of the light beam relative to the scanning interval in other words, it is well known that the latitude for the BAE method is smaller than the latitude for the IAE method.
Therefore, an optimal spot diameter and light beam strength must be set for the individual image forming methods.
FIG. 23 is a graph showing the relationship between the distribution of the energy of a light beam, the photosensitive characteristic of a photosensitive body, and the potential distribution on a photosensitive body. As is apparent from FIG. 23, when the photosensitive characteristic of the photosensitive body is linear, the energy distribution of the light beam is directly reflected by the potential distribution on the photosensitive body.
In electrophotography, the step whereat the transferring material P that has passed through the fixing portion is separated from the face of the photosensitive body 1 mainly employs electrostatics, machines, or the curvature of the photosensitive body 1 and the effect of gravity on the transfer material P. In either case, the transfer material P can be easily separated when the adhesive force between the transferring material P and the photosensitive body 1 is small. This greatly depends on the sealing condition at the portion whereat the transfer material P contacts the photosensitive body 1. The adhesive force is especially increased for a smoother transfer material P, such as an OHP sheet, rather than a fiber material, such as paper.
To improve the above shortcoming, it is effective for appropriate toner fogging to occur on the face of the photosensitive body 1.
Further, as for the cleaning means 6, the step whereat the surface of the photosensitive body 1 is cleaned by the cleaning blade is initiated when the blade closely contacts the surface of the photosensitive body 1 and smoothly slides across the surface.
That is, when the blade is pressed against the surface of the photosensitive body 1 too tightly, the friction between the two parts is increased and may be sufficient to cause great damage to the photosensitive body and the blade, or to warp the blade. And when the blade slides too smoothly and freely along the surface of the photosensitive body 1, the surface will be inadequately cleaned.
Specifically, for the conventional setting of the cleaning property, when the efficiency of the transfer of an image from the face of the photosensitive body 1 to the transfer material P is increased, when the time (idling time) is extended for cleaning the photosensitive body 1 without the developing process being performed, or when images, such as solid blank images, for which only a small amount of toner is required are continued, several defects appear: the blade contacts the surface of the photosensitive body 1 too closely and is warped, or a cleaning failure occurs.
Therefore, in order to satisfactorily continue the blade cleaning process, appropriate fogging is required on the face of the photosensitive body 1.
At the step for cleaning the surface of the photosensitive body 1 using a fur brush, when there is no fogging across the entire face of the photosensitive body 1, the fur brush strikes the hard surface of the photosensitive body 1 directly, and this adverse procedure may be accompanied by the deterioration of the fur brush. Therefore, in this case also, it is effective for appropriate fogging to be present on the face of the photosensitive body 1.
However, while i) when the fogging on the face of the photosensitive body 1 attains a constant level, as is described above, the separation of the transfer material P from the photosensitive body 1 and the cleaning of the photosensitive body 1 are improved, ii) it is demanded that the fogging be reduced as much as possible to maintain high image quality and to reduce the consumption of toner. Thus, an increase in the latitude is requested.